The calculation itself is simple subtraction
Counting from 14 September 2026 to 31 December 2026 gives 108 days remaining (15 full weeks) — the target date minus the reference date, in whole calendar days, with no time-of-day or timezone component involved.
A second example, a fixed annual date
Counting from the same 14 September 2026 to 26 January 2027 (a fixed annual date) gives 134 days remaining. The calculation doesn't care what the date represents — it's the same subtraction regardless of whether the target is a deadline, a holiday, or any other fixed point on the calendar.
What happens once the date has passed
Counting from 14 September 2026 to 1 January 2026 — a date already behind the reference point — gives -256 days, displayed as "256 days ago" rather than a negative number. The underlying math is identical either direction; only the display framing changes based on the sign of the result.
Why "count from" doesn't have to be today
The reference date defaults to today, but can be changed to any date — useful for checking what a countdown would have shown on a past date, or for planning around a future reference point rather than the current date.